For more than a month, Satna district of Madhya Pradesh received deficient rainfall. But suddenly in two days, 400 mm of intense downpour triggered severe floods in the entire district. In a normal year, the district would have received this much rain spanning over two months. In fact, “extreme” spells of rain—defined as rainfall above 204.5 mm in 24 hours by the India Meteorological Department (IMD)—have flooded several places across 18 states in June and July. Though deaths due to lightning are common during monsoon months, it was unprecedentedly high for four days. Between July 31 and August 3, at least 56 people died from lightning in different parts of Odisha. At least 400 people have been killed by lightning in eight states since the monsoon season began in June. The summer monsoon this year has also been unusually strong in China. It brought intense rainfall and caused flash floods, killing 400 people and displacing 100,000 in July. In early August, severe thunderstorms wreaked havoc in parts of the US.
Similar incidences of intense storms, cloudbursts, and flooding are becoming the norm worldwide. But scientists find it difficult to predict their onset despite advancement in weather models and satellite technology. A consensus has been building up that these uncertainties in predictive capability could be due to a missing link—clouds.
“Clouds are the carriers of weather, so to speak. They enable hydrological cycles and are responsible for almost all precipitation,” says Thara Prabhakaran, chief project scientist of Physics and Dynamics of Tropical Clouds programme at the Indian Institute of Tropical Meteorology in Pune. They also act as the mediator between global climatic systems and their local manifestation. Yet, scientists know little about them, and barely understand how they affect weather, let alone climate cycles. As extreme and freak weather events become frequent with rising temperatures, scientists are scurrying to crack the mysteries behind clouds. So far, most scientists have zeroed in on one factor that plays a crucial role in the formation and evolution of clouds and thereby dictates its influence on weather—it is the aerosol.
Hiding behind a complicated-sounding name, aerosols are microscopic organic and inorganic particles that are constantly being released into the atmosphere. These could be natural, such as dust, volcanic ash, and vapour emitted by plants, or man-made such as agricultural dust, vehicular exhaust, emissions from mines, and soot from thermal power plants. These microscopic pollutants act as the sites where water vapour condenses to form cloud droplets. “The influence of clouds on weather appears to start at this minutest level,” says Prabhakaran. For instance, the type of aerosols and their abundance in the atmosphere dictate the behaviour of cloud—whether it will result in precipitation, bounce back solar radiation into space or trap the radiated heat. Local meteorological factors further influence cloud behaviour.
Polluted air suppresses rainfall
Greater the number of aerosols, larger is the number of cloud droplets, says Daniel Rosenfeld, a professor at the Institute of Earth Sciences, Hebrew University, Jerusalem.
But more cloud droplets do not necessarily mean higher precipitation. As cloud water gets distributed among too many aerosols, they result in a large number of smaller droplets, which are slower to coalesce into bigger droplets that can fall as precipitation, he adds. In other words, polluted air often suppresses rainfall.
A study, published in Water Resources Research in 2008, corroborates Rosenfeld’s observation. To understand how dust and precipitation are related, researchers from University of Virginia, USA, and NASA Goddard Institute for Space Studies analysed aerosol indices, wind directions, and rainfall data of the Sahelian stretch in Central and North Africa for the period of 1996-2005. Their analysis shows that dust from the region was suppressing rainfall. The fine dust particles in the region resulted in a large number of seeding sites of cloud droplets, which prevented the formation of droplets of the size required for rainfall, the study notes. While this is usually true for shallow clouds, Rosenfeld says polluted clouds are capable of causing greater havoc.
Role in lightning, cloudburst
Polluted clouds at times grow to greater heights due to convection, says Rosenfeld. At this level, the behaviour of cloud droplets changes. They attain sufficient size to start coalescing into bigger droplets that can fall as precipitation.
Unfortunately, these clouds, known as cumulonimbus, are responsible for extreme events such as tornadoes, hailstorms, severe dust storms, squalls, and cloud bursts, says M Mohapatra, Head of the Regional Specialised Meteorological Centre at India Meteorological Department in Delhi.
If a cumulonimbus cloud, with a large number of aerosols, grows beyond the freezing level, it results in a large number of free ice pellets in the upper parts of clouds. The continuous and rapid vertical flows of water and ice pellets within clouds create an enormous amount of static energy, which in turn results in lightning and hailstones. Several studies have shown that volcanic eruptions are usually followed by bouts of lightning because of high levels of aerosol loading in the clouds.
This observation can be applied to understand why lightning-related deaths are common in certain places in India. Lightning is the leading weather-related cause of death in the country. Since 2000, over 30,000 people have died due to lightning strikes. Although no event-specific studies have been conducted to confirm this link, a comparison of the regions chronically affected by lightning with an aerosol emission map of India shows a strong correlation. Aerosol loading over the mainland is highest along the Gangetic plains in the northern and eastern parts of the country, in Central India and in the Deccan Plateau. These regions are home to most polluting industries. People here also burn organic and farm waste on a large scale. And these are also the regions that report a maximum number of lightning deaths.
Besides, tall clouds serve as a vertical pathway for energy flow. “This is what we feel as turbulence while travelling in an aircraft that moves through a cloud,” says Mohapatra. The water droplets and ice within clouds constantly and rapidly move up and down and undergo a lot of collisions, making them extremely volatile and dangerous. Aerosols can have an invigorating effect on this volatility. A trigger is all that is required for the entire water content of the cloud to be unleashed at once, he adds. This is the reason tall clouds often result in cloudbursts as they move along the Himalayan slopes.
Perfect recipe for tornadoes
Still, extreme precipitation is just one of the possible side-effects of cumulonimbus clouds. They can generate high wind speeds. An abundance of aerosol particles in such a cloud system can extend their size and lifespan by delaying rains and causing extreme storms when the rains finally arrive. This results in extreme events such as tornadoes, dust storms, and squalls, says Mohapatra.
A recent study by researchers from the University of Texas at Austin in collaboration with the University of Colorado Boulder and NASA’s Jet Propulsion Laboratory validates Mohapatra’s caution. The researchers analysed geostationary satellite data of 2,430 cloud systems and concluded that aerosol concentration could increase the lifespan of storm systems by as much as three to 24 hours, depending on local meteorological conditions. “High concentrations of aerosol particles cause a reduction in droplet size, which could delay precipitation, and in the case of storm clouds we have seen that it contributes to the extension of the life and intensity of the storm,” says Sudip Chakraborty, lead author of the paper that was published in the Proceedings of National Academy of Sciences in June this year.
While there is no dearth of hypotheses that say cumulonimbus clouds mostly result in extreme weather events, the study led by Chakraborty is the only study that shows the role of aerosols in altering the cloud’s properties.
The main reason for this is the dearth of data regarding cloud properties, says Sagnik Dey, Assistant Professor at the Centre for Atmospheric Sciences, IIT-Delhi. As data quality and quantity both improve with the availability of more sophisticated and detailed satellite imagery, researchers are taking more interest in studying how changing cloud properties are resulting in freak and unusual weather events.
(The views expressed are strictly those of Down to Earth.)